This invention relates, in general, to the speed control of prime movers and, in particular, to the speed control of prime movers during proportional speed, overspeed and emergency overspeed conditions.
Prime movers operate under the control of large in-line fluid control valves which regulate the flow of motive fluid into the prime mover. These fluid control valves may include a stop valve and a control valve positioned in series in a steam header upstream from the steam turbine. The control valve may be set to any intermediate position between full open and full closed, whereas the stop valve is usually full open or full closed. The earliest speed control devices for prime movers were strictly mechanical and were called fly-ball governors. As the size of prime movers increased, the fly-ball governor was enhanced by fluid operated relays and so mechanical hydraulic control entered the marketplace.
Mechanical hydraulic control is gradually being replaced by an electrohydraulic control which replaces fly-ball governors with a magnetic pick-up and toothwheel to provide speed feedback information to an electric control.
U.S. Pat. No. 3,242,346 to Skoubo shows the use of a silicon controlled rectifier to be used primarily as an overspeed control for a prime mover.
Although primary overspeed control is capable of sensing normal changes in speed sufficiently fast to maintain control of turbine speed, certain failure-related conditions are capable of producing speed changes which occur too rapidly for normal control, or even an overspeed trip device, to timely react. For example, if a main control valve is stuck open upon the occurrence of a substantial load shed, the turbine speed, instead of responding to the normal control signals tending to reduce the turbine speed, begins a rapid and substantially constant speed increase. If left unchecked, this speed increase can exceed the overspeed trip limit of about 110 percent in about 0.5 seconds for a full load rejection. By the time a conventional stop valve can be actuated in response to this condition, the turbine speed may increase well beyond the overspeed trip limit.